Lennart Poettering, the author of systemd, has announced: "I just put a first version of a wiki document together that lists a couple of easy optimizations to get your boot times down to [less than] 2s. It also includes a list of suggested things to hack on to get even quicker boot-ups."

LXDE is quite good on that front if you don't mind your DE being fairly barebones. (Just make sure you check "Preferences > Desktop Session Settings" to make sure no undesired GNOME/KDE/Xfce autostart components came along for the ride)